1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for internal combustion engines, and more particularly to an evaporative fuel-purging control system of this kind which controls the flow rate at which evaporative fuel is purged into the intake system of the engine.
2. Prior Art
Conventionally, evaporative fuel-purging control systems have been widely used in internal combustion engines, which operate to prevent evaporative fuel from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of an air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged amount is large, the air-fuel ratio of the mixture fluctuates. To prevent such fluctuations, there have been proposed the following systems:
(i) A purging gas flow rate control system which reduces the purging amount from the start of the engine immediately after refueling or fill-up until the speed of a vehicle in which the engine is installed reaches a predetermined value, and also reduces the purging amount after the vehicle speed has reached the predetermined value and until the accumulated time period during which the vehicle speed exceeds the predetermined value reaches a predetermined time period, to thereby prevent fluctuations in the air-fuel ratio due to purging immediately after a fill-up when a large amount of fuel vapor can be produced in the fuel tank (e.g. Japanese Provisional Patent Publication (Kokai) No. 63-111277);
(ii) An air-fuel ratio control system which effects purging of evaporative fuel in such a small amount as to cause almost no fluctuation of the air-fuel ratio, detects an amount of variation of an air-fuel ratio correction coefficient applied to feedback control of the air-fuel ratio, which is caused by the purging, forecast from the detected variation amount a value of the air-fuel ratio correction coefficient which should be assumed when the purging amount is large, and applies the forecast value as the air-fuel ratio correction coefficient in the feedback control when the actual purging amount becomes large, so as to reduce the fuel amount supplied to the engine, whereby fluctuations in the air-fuel ratio can be suppressed even when the purging amount is large (e.g. Japanese Provisional Patent Publication (Kokai) No. 62-131962);
(iii) A purging gas flow rate control system which employs a plurality of purge control valves, and calculates a forecast value of an air-fuel ratio correction coefficient to be applied during large-amount purging, based upon values of the correction coefficient assumed during stoppage of the purging and during small-amount purging, and inhibits large-amount purging when the forecast value exceeds a predetermined value (e.g. Japanese Provisional Patent Publication (Kokai) No. 62-233466).
When a throttle value of an internal combustion is fully closed or almost fully closed over a long time period, for example, at parking idle or deceleration, an amount of intake air supplied to the engine is small, and accordingly, purging of evaporative fuel to the intake system is stopped to prevent fluctuations in the air-fuel ratio. However, even when the throttle valve is fully closed or almost fully closed, evaporative fuel flows into the canister from the fuel tank. Therefore, there is a possibility that the canister becomes saturated with evaporated fuel adsorbed therein.
As a result, when the throttle valve is opened and fuel purging to the intake system is resumed while the canister is in such a saturated state, a large amount of evaporative fuel is supplied to the engine from the canister via the intake system so that the air-fuel ratio of the mixture largely changes to the rich side, which can result in a misfire. Furthermore, such enriched mixture will cause degraded accelerability, because the engine rotational speed cannot be raised even if the accelerator pedal is stepped on.
The above system (i) merely reduces the purging amount under predetermined conditions determined by the vehicle speed and the predetermined time period after a fill-up. The systems (ii) and (iii) merely attempt to suppress fluctuations in the air-fuel ratio by forecasting a value of the air-fuel ratio correction coefficient to be assumed when the purging amount is large, from an amount of variation of the air-fuel ratio correction coefficient during small-amount purging or during stoppage of the purging. Therefore, the systems (i)-(iii) cannot solve the above-described problems encountered when the purging is resumed after stoppage of the purging.